1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 6 <link rel="stylesheet" href="llvm.css" type="text/css"> 7 <title>LLVM Coding Standards</title> 8 </head> 9 <body> 10 11 <h1> 12 LLVM Coding Standards 13 </h1> 14 15 <ol> 16 <li><a href="#introduction">Introduction</a></li> 17 <li><a href="#mechanicalissues">Mechanical Source Issues</a> 18 <ol> 19 <li><a href="#sourceformating">Source Code Formatting</a> 20 <ol> 21 <li><a href="#scf_commenting">Commenting</a></li> 22 <li><a href="#scf_commentformat">Comment Formatting</a></li> 23 <li><a href="#scf_includes"><tt>#include</tt> Style</a></li> 24 <li><a href="#scf_codewidth">Source Code Width</a></li> 25 <li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li> 26 <li><a href="#scf_indentation">Indent Code Consistently</a></li> 27 </ol></li> 28 <li><a href="#compilerissues">Compiler Issues</a> 29 <ol> 30 <li><a href="#ci_warningerrors">Treat Compiler Warnings Like 31 Errors</a></li> 32 <li><a href="#ci_portable_code">Write Portable Code</a></li> 33 <li><a href="#ci_rtti_exceptions">Do not use RTTI or Exceptions</a></li> 34 <li><a href="#ci_static_ctors">Do not use Static Constructors</a></li> 35 <li><a href="#ci_class_struct">Use of <tt>class</tt>/<tt>struct</tt> Keywords</a></li> 36 </ol></li> 37 </ol></li> 38 <li><a href="#styleissues">Style Issues</a> 39 <ol> 40 <li><a href="#macro">The High-Level Issues</a> 41 <ol> 42 <li><a href="#hl_module">A Public Header File <b>is</b> a 43 Module</a></li> 44 <li><a href="#hl_dontinclude"><tt>#include</tt> as Little as Possible</a></li> 45 <li><a href="#hl_privateheaders">Keep "internal" Headers 46 Private</a></li> 47 <li><a href="#hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify 48 Code</a></li> 49 <li><a href="#hl_else_after_return">Don't use <tt>else</tt> after a 50 <tt>return</tt></a></li> 51 <li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate 52 Functions</a></li> 53 </ol></li> 54 <li><a href="#micro">The Low-Level Issues</a> 55 <ol> 56 <li><a href="#ll_naming">Name Types, Functions, Variables, and Enumerators Properly</a></li> 57 <li><a href="#ll_assert">Assert Liberally</a></li> 58 <li><a href="#ll_ns_std">Do not use '<tt>using namespace std</tt>'</a></li> 59 <li><a href="#ll_virtual_anch">Provide a virtual method anchor for 60 classes in headers</a></li> 61 <li><a href="#ll_end">Don't evaluate <tt>end()</tt> every time through a 62 loop</a></li> 63 <li><a href="#ll_iostream"><tt>#include <iostream></tt> is 64 <em>forbidden</em></a></li> 65 <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a></li> 66 <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li> 67 </ol></li> 68 69 <li><a href="#nano">Microscopic Details</a> 70 <ol> 71 <li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li> 72 <li><a href="#micro_preincrement">Prefer Preincrement</a></li> 73 <li><a href="#micro_namespaceindent">Namespace Indentation</a></li> 74 <li><a href="#micro_anonns">Anonymous Namespaces</a></li> 75 </ol></li> 76 77 78 </ol></li> 79 <li><a href="#seealso">See Also</a></li> 80 </ol> 81 82 <div class="doc_author"> 83 <p>Written by <a href="mailto:sabre (a] nondot.org">Chris Lattner</a></p> 84 </div> 85 86 87 <!-- *********************************************************************** --> 88 <h2><a name="introduction">Introduction</a></h2> 89 <!-- *********************************************************************** --> 90 91 <div> 92 93 <p>This document attempts to describe a few coding standards that are being used 94 in the LLVM source tree. Although no coding standards should be regarded as 95 absolute requirements to be followed in all instances, coding standards are 96 particularly important for large-scale code bases that follow a library-based 97 design (like LLVM).</p> 98 99 <p>This document intentionally does not prescribe fixed standards for religious 100 issues such as brace placement and space usage. For issues like this, follow 101 the golden rule:</p> 102 103 <blockquote> 104 105 <p><b><a name="goldenrule">If you are extending, enhancing, or bug fixing 106 already implemented code, use the style that is already being used so that the 107 source is uniform and easy to follow.</a></b></p> 108 109 </blockquote> 110 111 <p>Note that some code bases (e.g. libc++) have really good reasons to deviate 112 from the coding standards. In the case of libc++, this is because the naming 113 and other conventions are dictated by the C++ standard. If you think there is 114 a specific good reason to deviate from the standards here, please bring it up 115 on the LLVMdev mailing list.</p> 116 117 <p>There are some conventions that are not uniformly followed in the code base 118 (e.g. the naming convention). This is because they are relatively new, and a 119 lot of code was written before they were put in place. Our long term goal is 120 for the entire codebase to follow the convention, but we explicitly <em>do 121 not</em> want patches that do large-scale reformating of existing code. OTOH, 122 it is reasonable to rename the methods of a class if you're about to change it 123 in some other way. Just do the reformating as a separate commit from the 124 functionality change. </p> 125 126 <p>The ultimate goal of these guidelines is the increase readability and 127 maintainability of our common source base. If you have suggestions for topics to 128 be included, please mail them to <a 129 href="mailto:sabre (a] nondot.org">Chris</a>.</p> 130 131 </div> 132 133 <!-- *********************************************************************** --> 134 <h2> 135 <a name="mechanicalissues">Mechanical Source Issues</a> 136 </h2> 137 <!-- *********************************************************************** --> 138 139 <div> 140 141 <!-- ======================================================================= --> 142 <h3> 143 <a name="sourceformating">Source Code Formatting</a> 144 </h3> 145 146 <div> 147 148 <!-- _______________________________________________________________________ --> 149 <h4> 150 <a name="scf_commenting">Commenting</a> 151 </h4> 152 153 <div> 154 155 <p>Comments are one critical part of readability and maintainability. Everyone 156 knows they should comment their code, and so should you. When writing comments, 157 write them as English prose, which means they should use proper capitalization, 158 punctuation, etc. Aim to describe what a code is trying to do and why, not 159 "how" it does it at a micro level. Here are a few critical things to 160 document:</p> 161 162 <h5>File Headers</h5> 163 164 <div> 165 166 <p>Every source file should have a header on it that describes the basic 167 purpose of the file. If a file does not have a header, it should not be 168 checked into the tree. The standard header looks like this:</p> 169 170 <div class="doc_code"> 171 <pre> 172 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===// 173 // 174 // The LLVM Compiler Infrastructure 175 // 176 // This file is distributed under the University of Illinois Open Source 177 // License. See LICENSE.TXT for details. 178 // 179 //===----------------------------------------------------------------------===// 180 // 181 // This file contains the declaration of the Instruction class, which is the 182 // base class for all of the VM instructions. 183 // 184 //===----------------------------------------------------------------------===// 185 </pre> 186 </div> 187 188 <p>A few things to note about this particular format: The "<tt>-*- C++ 189 -*-</tt>" string on the first line is there to tell Emacs that the source file 190 is a C++ file, not a C file (Emacs assumes <tt>.h</tt> files are C files by default). 191 Note that this tag is not necessary in <tt>.cpp</tt> files. The name of the file is also 192 on the first line, along with a very short description of the purpose of the 193 file. This is important when printing out code and flipping though lots of 194 pages.</p> 195 196 <p>The next section in the file is a concise note that defines the license 197 that the file is released under. This makes it perfectly clear what terms the 198 source code can be distributed under and should not be modified in any way.</p> 199 200 <p>The main body of the description does not have to be very long in most cases. 201 Here it's only two lines. If an algorithm is being implemented or something 202 tricky is going on, a reference to the paper where it is published should be 203 included, as well as any notes or "gotchas" in the code to watch out for.</p> 204 205 </div> 206 207 <h5>Class overviews</h5> 208 209 <p>Classes are one fundamental part of a good object oriented design. As such, 210 a class definition should have a comment block that explains what the class is 211 used for and how it works. Every non-trivial class is expected to have a 212 doxygen comment block.</p> 213 214 215 <h5>Method information</h5> 216 217 <div> 218 219 <p>Methods defined in a class (as well as any global functions) should also be 220 documented properly. A quick note about what it does and a description of the 221 borderline behaviour is all that is necessary here (unless something 222 particularly tricky or insidious is going on). The hope is that people can 223 figure out how to use your interfaces without reading the code itself.</p> 224 225 <p>Good things to talk about here are what happens when something unexpected 226 happens: does the method return null? Abort? Format your hard disk?</p> 227 228 </div> 229 230 </div> 231 232 <!-- _______________________________________________________________________ --> 233 <h4> 234 <a name="scf_commentformat">Comment Formatting</a> 235 </h4> 236 237 <div> 238 239 <p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space, 240 require less typing, don't have nesting problems, etc. There are a few cases 241 when it is useful to use C style (<tt>/* */</tt>) comments however:</p> 242 243 <ol> 244 <li>When writing C code: Obviously if you are writing C code, use C style 245 comments.</li> 246 <li>When writing a header file that may be <tt>#include</tt>d by a C source 247 file.</li> 248 <li>When writing a source file that is used by a tool that only accepts C 249 style comments.</li> 250 </ol> 251 252 <p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>. 253 These nest properly and are better behaved in general than C style comments.</p> 254 255 </div> 256 257 <!-- _______________________________________________________________________ --> 258 <h4> 259 <a name="scf_includes"><tt>#include</tt> Style</a> 260 </h4> 261 262 <div> 263 264 <p>Immediately after the <a href="#scf_commenting">header file comment</a> (and 265 include guards if working on a header file), the <a 266 href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the 267 file should be listed. We prefer these <tt>#include</tt>s to be listed in this 268 order:</p> 269 270 <ol> 271 <li><a href="#mmheader">Main Module Header</a></li> 272 <li><a href="#hl_privateheaders">Local/Private Headers</a></li> 273 <li><tt>llvm/*</tt></li> 274 <li><tt>llvm/Analysis/*</tt></li> 275 <li><tt>llvm/Assembly/*</tt></li> 276 <li><tt>llvm/Bitcode/*</tt></li> 277 <li><tt>llvm/CodeGen/*</tt></li> 278 <li>...</li> 279 <li><tt>Support/*</tt></li> 280 <li><tt>Config/*</tt></li> 281 <li>System <tt>#includes</tt></li> 282 </ol> 283 284 <p>and each category should be sorted by name.</p> 285 286 <p><a name="mmheader">The "Main Module Header"</a> file applies to <tt>.cpp</tt> files 287 which implement an interface defined by a <tt>.h</tt> file. This <tt>#include</tt> 288 should always be included <b>first</b> regardless of where it lives on the file 289 system. By including a header file first in the <tt>.cpp</tt> files that implement the 290 interfaces, we ensure that the header does not have any hidden dependencies 291 which are not explicitly #included in the header, but should be. It is also a 292 form of documentation in the <tt>.cpp</tt> file to indicate where the interfaces it 293 implements are defined.</p> 294 295 </div> 296 297 <!-- _______________________________________________________________________ --> 298 <h4> 299 <a name="scf_codewidth">Source Code Width</a> 300 </h4> 301 302 <div> 303 304 <p>Write your code to fit within 80 columns of text. This helps those of us who 305 like to print out code and look at your code in an xterm without resizing 306 it.</p> 307 308 <p>The longer answer is that there must be some limit to the width of the code 309 in order to reasonably allow developers to have multiple files side-by-side in 310 windows on a modest display. If you are going to pick a width limit, it is 311 somewhat arbitrary but you might as well pick something standard. Going with 312 90 columns (for example) instead of 80 columns wouldn't add any significant 313 value and would be detrimental to printing out code. Also many other projects 314 have standardized on 80 columns, so some people have already configured their 315 editors for it (vs something else, like 90 columns).</p> 316 317 <p>This is one of many contentious issues in coding standards, but it is not up 318 for debate.</p> 319 320 </div> 321 322 <!-- _______________________________________________________________________ --> 323 <h4> 324 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a> 325 </h4> 326 327 <div> 328 329 <p>In all cases, prefer spaces to tabs in source files. People have different 330 preferred indentation levels, and different styles of indentation that they 331 like; this is fine. What isn't fine is that different editors/viewers expand 332 tabs out to different tab stops. This can cause your code to look completely 333 unreadable, and it is not worth dealing with.</p> 334 335 <p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the 336 style of existing code if you are modifying and extending it. If you like four 337 spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code 338 with two spaces of indentation. Also, do not reindent a whole source file: it 339 makes for incredible diffs that are absolutely worthless.</p> 340 341 </div> 342 343 <!-- _______________________________________________________________________ --> 344 <h4> 345 <a name="scf_indentation">Indent Code Consistently</a> 346 </h4> 347 348 <div> 349 350 <p>Okay, in your first year of programming you were told that indentation is 351 important. If you didn't believe and internalize this then, now is the time. 352 Just do it.</p> 353 354 </div> 355 356 </div> 357 358 <!-- ======================================================================= --> 359 <h3> 360 <a name="compilerissues">Compiler Issues</a> 361 </h3> 362 363 <div> 364 365 <!-- _______________________________________________________________________ --> 366 <h4> 367 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a> 368 </h4> 369 370 <div> 371 372 <p>If your code has compiler warnings in it, something is wrong — you 373 aren't casting values correctly, your have "questionable" constructs in your 374 code, or you are doing something legitimately wrong. Compiler warnings can 375 cover up legitimate errors in output and make dealing with a translation unit 376 difficult.</p> 377 378 <p>It is not possible to prevent all warnings from all compilers, nor is it 379 desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides 380 a good thorough set of warnings, and stick to it. At least in the case of 381 <tt>gcc</tt>, it is possible to work around any spurious errors by changing the 382 syntax of the code slightly. For example, a warning that annoys me occurs when 383 I write code like this:</p> 384 385 <div class="doc_code"> 386 <pre> 387 if (V = getValue()) { 388 ... 389 } 390 </pre> 391 </div> 392 393 <p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt> 394 operator, and that I probably mistyped it. In most cases, I haven't, and I 395 really don't want the spurious errors. To fix this particular problem, I 396 rewrite the code like this:</p> 397 398 <div class="doc_code"> 399 <pre> 400 if ((V = getValue())) { 401 ... 402 } 403 </pre> 404 </div> 405 406 <p>which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can 407 be fixed by massaging the code appropriately.</p> 408 409 </div> 410 411 <!-- _______________________________________________________________________ --> 412 <h4> 413 <a name="ci_portable_code">Write Portable Code</a> 414 </h4> 415 416 <div> 417 418 <p>In almost all cases, it is possible and within reason to write completely 419 portable code. If there are cases where it isn't possible to write portable 420 code, isolate it behind a well defined (and well documented) interface.</p> 421 422 <p>In practice, this means that you shouldn't assume much about the host 423 compiler, and Visual Studio tends to be the lowest common denominator. 424 If advanced features are used, they should only be an implementation detail of 425 a library which has a simple exposed API, and preferably be buried in 426 libSystem.</p> 427 428 </div> 429 430 <!-- _______________________________________________________________________ --> 431 <h4> 432 <a name="ci_rtti_exceptions">Do not use RTTI or Exceptions</a> 433 </h4> 434 <div> 435 436 <p>In an effort to reduce code and executable size, LLVM does not use RTTI 437 (e.g. <tt>dynamic_cast<></tt>) or exceptions. These two language features 438 violate the general C++ principle of <i>"you only pay for what you use"</i>, 439 causing executable bloat even if exceptions are never used in the code base, or 440 if RTTI is never used for a class. Because of this, we turn them off globally 441 in the code.</p> 442 443 <p>That said, LLVM does make extensive use of a hand-rolled form of RTTI that 444 use templates like <a href="ProgrammersManual.html#isa"><tt>isa<></tt>, 445 <tt>cast<></tt>, and <tt>dyn_cast<></tt></a>. This form of RTTI is 446 opt-in and can be added to any class. It is also substantially more efficient 447 than <tt>dynamic_cast<></tt>.</p> 448 449 </div> 450 451 <!-- _______________________________________________________________________ --> 452 <h4> 453 <a name="ci_static_ctors">Do not use Static Constructors</a> 454 </h4> 455 <div> 456 457 <p>Static constructors and destructors (e.g. global variables whose types have 458 a constructor or destructor) should not be added to the code base, and should be 459 removed wherever possible. Besides <a 460 href="http://yosefk.com/c++fqa/ctors.html#fqa-10.12">well known problems</a> 461 where the order of initialization is undefined between globals in different 462 source files, the entire concept of static constructors is at odds with the 463 common use case of LLVM as a library linked into a larger application.</p> 464 465 <p>Consider the use of LLVM as a JIT linked into another application (perhaps 466 for <a href="http://llvm.org/Users.html">OpenGL, custom languages</a>, 467 <a href="http://llvm.org/devmtg/2010-11/Gritz-OpenShadingLang.pdf">shaders in 468 movies</a>, etc). Due to the design of static constructors, they must be 469 executed at startup time of the entire application, regardless of whether or 470 how LLVM is used in that larger application. There are two problems with 471 this:</p> 472 473 <ol> 474 <li>The time to run the static constructors impacts startup time of 475 applications — a critical time for GUI apps, among others.</li> 476 477 <li>The static constructors cause the app to pull many extra pages of memory 478 off the disk: both the code for the constructor in each <tt>.o</tt> file and 479 the small amount of data that gets touched. In addition, touched/dirty pages 480 put more pressure on the VM system on low-memory machines.</li> 481 </ol> 482 483 <p>We would really like for there to be zero cost for linking in an additional 484 LLVM target or other library into an application, but static constructors 485 violate this goal.</p> 486 487 <p>That said, LLVM unfortunately does contain static constructors. It would be 488 a <a href="http://llvm.org/PR11944">great project</a> for someone to purge all 489 static constructors from LLVM, and then enable the 490 <tt>-Wglobal-constructors</tt> warning flag (when building with Clang) to ensure 491 we do not regress in the future. 492 </p> 493 494 </div> 495 496 <!-- _______________________________________________________________________ --> 497 <h4> 498 <a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a> 499 </h4> 500 <div> 501 502 <p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost 503 interchangeably. The only difference is when they are used to declare a class: 504 <tt>class</tt> makes all members private by default while <tt>struct</tt> makes 505 all members public by default.</p> 506 507 <p>Unfortunately, not all compilers follow the rules and some will generate 508 different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to 509 declare the symbol. This can lead to problems at link time.</p> 510 511 <p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless 512 <b>all</b> members are public and the type is a C++ 513 <a href="http://en.wikipedia.org/wiki/Plain_old_data_structure">POD</a> type, in 514 which case <tt>struct</tt> is allowed.</p> 515 516 </div> 517 518 </div> 519 520 </div> 521 522 <!-- *********************************************************************** --> 523 <h2> 524 <a name="styleissues">Style Issues</a> 525 </h2> 526 <!-- *********************************************************************** --> 527 528 <div> 529 530 <!-- ======================================================================= --> 531 <h3> 532 <a name="macro">The High-Level Issues</a> 533 </h3> 534 <!-- ======================================================================= --> 535 536 <div> 537 538 <!-- _______________________________________________________________________ --> 539 <h4> 540 <a name="hl_module">A Public Header File <b>is</b> a Module</a> 541 </h4> 542 543 <div> 544 545 <p>C++ doesn't do too well in the modularity department. There is no real 546 encapsulation or data hiding (unless you use expensive protocol classes), but it 547 is what we have to work with. When you write a public header file (in the LLVM 548 source tree, they live in the top level "<tt>include</tt>" directory), you are 549 defining a module of functionality.</p> 550 551 <p>Ideally, modules should be completely independent of each other, and their 552 header files should only <tt>#include</tt> the absolute minimum number of 553 headers possible. A module is not just a class, a function, or a 554 namespace: <a href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's 555 a collection of these</a> that defines an interface. This interface may be 556 several functions, classes, or data structures, but the important issue is how 557 they work together.</p> 558 559 <p>In general, a module should be implemented by one or more <tt>.cpp</tt> 560 files. Each of these <tt>.cpp</tt> files should include the header that defines 561 their interface first. This ensures that all of the dependences of the module 562 header have been properly added to the module header itself, and are not 563 implicit. System headers should be included after user headers for a 564 translation unit.</p> 565 566 </div> 567 568 <!-- _______________________________________________________________________ --> 569 <h4> 570 <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a> 571 </h4> 572 573 <div> 574 575 <p><tt>#include</tt> hurts compile time performance. Don't do it unless you 576 have to, especially in header files.</p> 577 578 <p>But wait! Sometimes you need to have the definition of a class to use it, or 579 to inherit from it. In these cases go ahead and <tt>#include</tt> that header 580 file. Be aware however that there are many cases where you don't need to have 581 the full definition of a class. If you are using a pointer or reference to a 582 class, you don't need the header file. If you are simply returning a class 583 instance from a prototyped function or method, you don't need it. In fact, for 584 most cases, you simply don't need the definition of a class. And not 585 <tt>#include</tt>'ing speeds up compilation.</p> 586 587 <p>It is easy to try to go too overboard on this recommendation, however. You 588 <b>must</b> include all of the header files that you are using — you can 589 include them either directly or indirectly (through another header file). To 590 make sure that you don't accidentally forget to include a header file in your 591 module header, make sure to include your module header <b>first</b> in the 592 implementation file (as mentioned above). This way there won't be any hidden 593 dependencies that you'll find out about later.</p> 594 595 </div> 596 597 <!-- _______________________________________________________________________ --> 598 <h4> 599 <a name="hl_privateheaders">Keep "Internal" Headers Private</a> 600 </h4> 601 602 <div> 603 604 <p>Many modules have a complex implementation that causes them to use more than 605 one implementation (<tt>.cpp</tt>) file. It is often tempting to put the 606 internal communication interface (helper classes, extra functions, etc) in the 607 public module header file. Don't do this!</p> 608 609 <p>If you really need to do something like this, put a private header file in 610 the same directory as the source files, and include it locally. This ensures 611 that your private interface remains private and undisturbed by outsiders.</p> 612 613 <p>Note however, that it's okay to put extra implementation methods in a public 614 class itself. Just make them private (or protected) and all is well.</p> 615 616 </div> 617 618 <!-- _______________________________________________________________________ --> 619 <h4> 620 <a name="hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify Code</a> 621 </h4> 622 623 <div> 624 625 <p>When reading code, keep in mind how much state and how many previous 626 decisions have to be remembered by the reader to understand a block of code. 627 Aim to reduce indentation where possible when it doesn't make it more difficult 628 to understand the code. One great way to do this is by making use of early 629 exits and the <tt>continue</tt> keyword in long loops. As an example of using 630 an early exit from a function, consider this "bad" code:</p> 631 632 <div class="doc_code"> 633 <pre> 634 Value *DoSomething(Instruction *I) { 635 if (!isa<TerminatorInst>(I) && 636 I->hasOneUse() && SomeOtherThing(I)) { 637 ... some long code .... 638 } 639 640 return 0; 641 } 642 </pre> 643 </div> 644 645 <p>This code has several problems if the body of the '<tt>if</tt>' is large. 646 When you're looking at the top of the function, it isn't immediately clear that 647 this <em>only</em> does interesting things with non-terminator instructions, and 648 only applies to things with the other predicates. Second, it is relatively 649 difficult to describe (in comments) why these predicates are important because 650 the <tt>if</tt> statement makes it difficult to lay out the comments. Third, 651 when you're deep within the body of the code, it is indented an extra level. 652 Finally, when reading the top of the function, it isn't clear what the result is 653 if the predicate isn't true; you have to read to the end of the function to know 654 that it returns null.</p> 655 656 <p>It is much preferred to format the code like this:</p> 657 658 <div class="doc_code"> 659 <pre> 660 Value *DoSomething(Instruction *I) { 661 // Terminators never need 'something' done to them because ... 662 if (isa<TerminatorInst>(I)) 663 return 0; 664 665 // We conservatively avoid transforming instructions with multiple uses 666 // because goats like cheese. 667 if (!I->hasOneUse()) 668 return 0; 669 670 // This is really just here for example. 671 if (!SomeOtherThing(I)) 672 return 0; 673 674 ... some long code .... 675 } 676 </pre> 677 </div> 678 679 <p>This fixes these problems. A similar problem frequently happens in <tt>for</tt> 680 loops. A silly example is something like this:</p> 681 682 <div class="doc_code"> 683 <pre> 684 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { 685 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) { 686 Value *LHS = BO->getOperand(0); 687 Value *RHS = BO->getOperand(1); 688 if (LHS != RHS) { 689 ... 690 } 691 } 692 } 693 </pre> 694 </div> 695 696 <p>When you have very, very small loops, this sort of structure is fine. But if 697 it exceeds more than 10-15 lines, it becomes difficult for people to read and 698 understand at a glance. The problem with this sort of code is that it gets very 699 nested very quickly. Meaning that the reader of the code has to keep a lot of 700 context in their brain to remember what is going immediately on in the loop, 701 because they don't know if/when the <tt>if</tt> conditions will have elses etc. 702 It is strongly preferred to structure the loop like this:</p> 703 704 <div class="doc_code"> 705 <pre> 706 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { 707 BinaryOperator *BO = dyn_cast<BinaryOperator>(II); 708 if (!BO) continue; 709 710 Value *LHS = BO->getOperand(0); 711 Value *RHS = BO->getOperand(1); 712 if (LHS == RHS) continue; 713 714 ... 715 } 716 </pre> 717 </div> 718 719 <p>This has all the benefits of using early exits for functions: it reduces 720 nesting of the loop, it makes it easier to describe why the conditions are true, 721 and it makes it obvious to the reader that there is no <tt>else</tt> coming up 722 that they have to push context into their brain for. If a loop is large, this 723 can be a big understandability win.</p> 724 725 </div> 726 727 <!-- _______________________________________________________________________ --> 728 <h4> 729 <a name="hl_else_after_return">Don't use <tt>else</tt> after a <tt>return</tt></a> 730 </h4> 731 732 <div> 733 734 <p>For similar reasons above (reduction of indentation and easier reading), 735 please do not use '<tt>else</tt>' or '<tt>else if</tt>' after something that 736 interrupts control flow — like <tt>return</tt>, <tt>break</tt>, 737 <tt>continue</tt>, <tt>goto</tt>, etc. For example, this is <em>bad</em>:</p> 738 739 <div class="doc_code"> 740 <pre> 741 case 'J': { 742 if (Signed) { 743 Type = Context.getsigjmp_bufType(); 744 if (Type.isNull()) { 745 Error = ASTContext::GE_Missing_sigjmp_buf; 746 return QualType(); 747 <b>} else { 748 break; 749 }</b> 750 } else { 751 Type = Context.getjmp_bufType(); 752 if (Type.isNull()) { 753 Error = ASTContext::GE_Missing_jmp_buf; 754 return QualType(); 755 <b>} else { 756 break; 757 }</b> 758 } 759 } 760 } 761 </pre> 762 </div> 763 764 <p>It is better to write it like this:</p> 765 766 <div class="doc_code"> 767 <pre> 768 case 'J': 769 if (Signed) { 770 Type = Context.getsigjmp_bufType(); 771 if (Type.isNull()) { 772 Error = ASTContext::GE_Missing_sigjmp_buf; 773 return QualType(); 774 } 775 } else { 776 Type = Context.getjmp_bufType(); 777 if (Type.isNull()) { 778 Error = ASTContext::GE_Missing_jmp_buf; 779 return QualType(); 780 } 781 } 782 <b>break;</b> 783 </pre> 784 </div> 785 786 <p>Or better yet (in this case) as:</p> 787 788 <div class="doc_code"> 789 <pre> 790 case 'J': 791 if (Signed) 792 Type = Context.getsigjmp_bufType(); 793 else 794 Type = Context.getjmp_bufType(); 795 796 if (Type.isNull()) { 797 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf : 798 ASTContext::GE_Missing_jmp_buf; 799 return QualType(); 800 } 801 <b>break;</b> 802 </pre> 803 </div> 804 805 <p>The idea is to reduce indentation and the amount of code you have to keep 806 track of when reading the code.</p> 807 808 </div> 809 810 <!-- _______________________________________________________________________ --> 811 <h4> 812 <a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a> 813 </h4> 814 815 <div> 816 817 <p>It is very common to write small loops that just compute a boolean value. 818 There are a number of ways that people commonly write these, but an example of 819 this sort of thing is:</p> 820 821 <div class="doc_code"> 822 <pre> 823 <b>bool FoundFoo = false;</b> 824 for (unsigned i = 0, e = BarList.size(); i != e; ++i) 825 if (BarList[i]->isFoo()) { 826 <b>FoundFoo = true;</b> 827 break; 828 } 829 830 <b>if (FoundFoo) {</b> 831 ... 832 } 833 </pre> 834 </div> 835 836 <p>This sort of code is awkward to write, and is almost always a bad sign. 837 Instead of this sort of loop, we strongly prefer to use a predicate function 838 (which may be <a href="#micro_anonns">static</a>) that uses 839 <a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer 840 the code to be structured like this:</p> 841 842 <div class="doc_code"> 843 <pre> 844 /// ListContainsFoo - Return true if the specified list has an element that is 845 /// a foo. 846 static bool ListContainsFoo(const std::vector<Bar*> &List) { 847 for (unsigned i = 0, e = List.size(); i != e; ++i) 848 if (List[i]->isFoo()) 849 return true; 850 return false; 851 } 852 ... 853 854 <b>if (ListContainsFoo(BarList)) {</b> 855 ... 856 } 857 </pre> 858 </div> 859 860 <p>There are many reasons for doing this: it reduces indentation and factors out 861 code which can often be shared by other code that checks for the same predicate. 862 More importantly, it <em>forces you to pick a name</em> for the function, and 863 forces you to write a comment for it. In this silly example, this doesn't add 864 much value. However, if the condition is complex, this can make it a lot easier 865 for the reader to understand the code that queries for this predicate. Instead 866 of being faced with the in-line details of how we check to see if the BarList 867 contains a foo, we can trust the function name and continue reading with better 868 locality.</p> 869 870 </div> 871 872 </div> 873 874 <!-- ======================================================================= --> 875 <h3> 876 <a name="micro">The Low-Level Issues</a> 877 </h3> 878 <!-- ======================================================================= --> 879 880 <div> 881 882 <!-- _______________________________________________________________________ --> 883 <h4> 884 <a name="ll_naming"> 885 Name Types, Functions, Variables, and Enumerators Properly 886 </a> 887 </h4> 888 889 <div> 890 891 <p>Poorly-chosen names can mislead the reader and cause bugs. We cannot stress 892 enough how important it is to use <em>descriptive</em> names. Pick names that 893 match the semantics and role of the underlying entities, within reason. Avoid 894 abbreviations unless they are well known. After picking a good name, make sure 895 to use consistent capitalization for the name, as inconsistency requires clients 896 to either memorize the APIs or to look it up to find the exact spelling.</p> 897 898 <p>In general, names should be in camel case (e.g. <tt>TextFileReader</tt> 899 and <tt>isLValue()</tt>). Different kinds of declarations have different 900 rules:</p> 901 902 <ul> 903 <li><p><b>Type names</b> (including classes, structs, enums, typedefs, etc) 904 should be nouns and start with an upper-case letter (e.g. 905 <tt>TextFileReader</tt>).</p></li> 906 907 <li><p><b>Variable names</b> should be nouns (as they represent state). The 908 name should be camel case, and start with an upper case letter (e.g. 909 <tt>Leader</tt> or <tt>Boats</tt>).</p></li> 910 911 <li><p><b>Function names</b> should be verb phrases (as they represent 912 actions), and command-like function should be imperative. The name should 913 be camel case, and start with a lower case letter (e.g. <tt>openFile()</tt> 914 or <tt>isFoo()</tt>).</p></li> 915 916 <li><p><b>Enum declarations</b> (e.g. <tt>enum Foo {...}</tt>) are types, so 917 they should follow the naming conventions for types. A common use for enums 918 is as a discriminator for a union, or an indicator of a subclass. When an 919 enum is used for something like this, it should have a <tt>Kind</tt> suffix 920 (e.g. <tt>ValueKind</tt>).</p></li> 921 922 <li><p><b>Enumerators</b> (e.g. <tt>enum { Foo, Bar }</tt>) and <b>public member 923 variables</b> should start with an upper-case letter, just like types. 924 Unless the enumerators are defined in their own small namespace or inside a 925 class, enumerators should have a prefix corresponding to the enum 926 declaration name. For example, <tt>enum ValueKind { ... };</tt> may contain 927 enumerators like <tt>VK_Argument</tt>, <tt>VK_BasicBlock</tt>, etc. 928 Enumerators that are just convenience constants are exempt from the 929 requirement for a prefix. For instance:</p> 930 931 <div class="doc_code"> 932 <pre> 933 enum { 934 MaxSize = 42, 935 Density = 12 936 }; 937 </pre> 938 </div> 939 </li> 940 941 </ul> 942 943 <p>As an exception, classes that mimic STL classes can have member names in 944 STL's style of lower-case words separated by underscores (e.g. <tt>begin()</tt>, 945 <tt>push_back()</tt>, and <tt>empty()</tt>).</p> 946 947 <p>Here are some examples of good and bad names:</p> 948 949 <div class="doc_code"> 950 <pre> 951 class VehicleMaker { 952 ... 953 Factory<Tire> F; // Bad -- abbreviation and non-descriptive. 954 Factory<Tire> Factory; // Better. 955 Factory<Tire> TireFactory; // Even better -- if VehicleMaker has more than one 956 // kind of factories. 957 }; 958 959 Vehicle MakeVehicle(VehicleType Type) { 960 VehicleMaker M; // Might be OK if having a short life-span. 961 Tire tmp1 = M.makeTire(); // Bad -- 'tmp1' provides no information. 962 Light headlight = M.makeLight("head"); // Good -- descriptive. 963 ... 964 } 965 </pre> 966 </div> 967 968 </div> 969 970 971 <!-- _______________________________________________________________________ --> 972 <h4> 973 <a name="ll_assert">Assert Liberally</a> 974 </h4> 975 976 <div> 977 978 <p>Use the "<tt>assert</tt>" macro to its fullest. Check all of your 979 preconditions and assumptions, you never know when a bug (not necessarily even 980 yours) might be caught early by an assertion, which reduces debugging time 981 dramatically. The "<tt><cassert></tt>" header file is probably already 982 included by the header files you are using, so it doesn't cost anything to use 983 it.</p> 984 985 <p>To further assist with debugging, make sure to put some kind of error message 986 in the assertion statement, which is printed if the assertion is tripped. This 987 helps the poor debugger make sense of why an assertion is being made and 988 enforced, and hopefully what to do about it. Here is one complete example:</p> 989 990 <div class="doc_code"> 991 <pre> 992 inline Value *getOperand(unsigned i) { 993 assert(i < Operands.size() && "getOperand() out of range!"); 994 return Operands[i]; 995 } 996 </pre> 997 </div> 998 999 <p>Here are more examples:</p> 1000 1001 <div class="doc_code"> 1002 <pre> 1003 assert(Ty->isPointerType() && "Can't allocate a non pointer type!"); 1004 1005 assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!"); 1006 1007 assert(idx < getNumSuccessors() && "Successor # out of range!"); 1008 1009 assert(V1.getType() == V2.getType() && "Constant types must be identical!"); 1010 1011 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!"); 1012 </pre> 1013 </div> 1014 1015 <p>You get the idea.</p> 1016 1017 <p>Please be aware that, when adding assert statements, not all compilers are aware of 1018 the semantics of the assert. In some places, asserts are used to indicate a piece of 1019 code that should not be reached. These are typically of the form:</p> 1020 1021 <div class="doc_code"> 1022 <pre> 1023 assert(0 && "Some helpful error message"); 1024 </pre> 1025 </div> 1026 1027 <p>When used in a function that returns a value, they should be followed with a return 1028 statement and a comment indicating that this line is never reached. This will prevent 1029 a compiler which is unable to deduce that the assert statement never returns from 1030 generating a warning.</p> 1031 1032 <div class="doc_code"> 1033 <pre> 1034 assert(0 && "Some helpful error message"); 1035 // Not reached 1036 return 0; 1037 </pre> 1038 </div> 1039 1040 <p>Another issue is that values used only by assertions will produce an "unused 1041 value" warning when assertions are disabled. For example, this code will 1042 warn:</p> 1043 1044 <div class="doc_code"> 1045 <pre> 1046 unsigned Size = V.size(); 1047 assert(Size > 42 && "Vector smaller than it should be"); 1048 1049 bool NewToSet = Myset.insert(Value); 1050 assert(NewToSet && "The value shouldn't be in the set yet"); 1051 </pre> 1052 </div> 1053 1054 <p>These are two interesting different cases. In the first case, the call to 1055 V.size() is only useful for the assert, and we don't want it executed when 1056 assertions are disabled. Code like this should move the call into the assert 1057 itself. In the second case, the side effects of the call must happen whether 1058 the assert is enabled or not. In this case, the value should be cast to void to 1059 disable the warning. To be specific, it is preferred to write the code like 1060 this:</p> 1061 1062 <div class="doc_code"> 1063 <pre> 1064 assert(V.size() > 42 && "Vector smaller than it should be"); 1065 1066 bool NewToSet = Myset.insert(Value); (void)NewToSet; 1067 assert(NewToSet && "The value shouldn't be in the set yet"); 1068 </pre> 1069 </div> 1070 1071 1072 </div> 1073 1074 <!-- _______________________________________________________________________ --> 1075 <h4> 1076 <a name="ll_ns_std">Do Not Use '<tt>using namespace std</tt>'</a> 1077 </h4> 1078 1079 <div> 1080 1081 <p>In LLVM, we prefer to explicitly prefix all identifiers from the standard 1082 namespace with an "<tt>std::</tt>" prefix, rather than rely on 1083 "<tt>using namespace std;</tt>".</p> 1084 1085 <p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes 1086 the namespace of any source file that <tt>#include</tt>s the header. This is 1087 clearly a bad thing.</p> 1088 1089 <p>In implementation files (e.g. <tt>.cpp</tt> files), the rule is more of a stylistic 1090 rule, but is still important. Basically, using explicit namespace prefixes 1091 makes the code <b>clearer</b>, because it is immediately obvious what facilities 1092 are being used and where they are coming from. And <b>more portable</b>, because 1093 namespace clashes cannot occur between LLVM code and other namespaces. The 1094 portability rule is important because different standard library implementations 1095 expose different symbols (potentially ones they shouldn't), and future revisions 1096 to the C++ standard will add more symbols to the <tt>std</tt> namespace. As 1097 such, we never use '<tt>using namespace std;</tt>' in LLVM.</p> 1098 1099 <p>The exception to the general rule (i.e. it's not an exception for 1100 the <tt>std</tt> namespace) is for implementation files. For example, all of 1101 the code in the LLVM project implements code that lives in the 'llvm' namespace. 1102 As such, it is ok, and actually clearer, for the <tt>.cpp</tt> files to have a 1103 '<tt>using namespace llvm;</tt>' directive at the top, after the 1104 <tt>#include</tt>s. This reduces indentation in the body of the file for source 1105 editors that indent based on braces, and keeps the conceptual context cleaner. 1106 The general form of this rule is that any <tt>.cpp</tt> file that implements 1107 code in any namespace may use that namespace (and its parents'), but should not 1108 use any others.</p> 1109 1110 </div> 1111 1112 <!-- _______________________________________________________________________ --> 1113 <h4> 1114 <a name="ll_virtual_anch"> 1115 Provide a Virtual Method Anchor for Classes in Headers 1116 </a> 1117 </h4> 1118 1119 <div> 1120 1121 <p>If a class is defined in a header file and has a v-table (either it has 1122 virtual methods or it derives from classes with virtual methods), it must 1123 always have at least one out-of-line virtual method in the class. Without 1124 this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file 1125 that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and 1126 increasing link times.</p> 1127 1128 </div> 1129 1130 <!-- _______________________________________________________________________ --> 1131 <h4> 1132 <a name="ll_end">Don't evaluate <tt>end()</tt> every time through a loop</a> 1133 </h4> 1134 1135 <div> 1136 1137 <p>Because C++ doesn't have a standard "<tt>foreach</tt>" loop (though it can be 1138 emulated with macros and may be coming in C++'0x) we end up writing a lot of 1139 loops that manually iterate from begin to end on a variety of containers or 1140 through other data structures. One common mistake is to write a loop in this 1141 style:</p> 1142 1143 <div class="doc_code"> 1144 <pre> 1145 BasicBlock *BB = ... 1146 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I) 1147 ... use I ... 1148 </pre> 1149 </div> 1150 1151 <p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>" 1152 every time through the loop. Instead of writing the loop like this, we strongly 1153 prefer loops to be written so that they evaluate it once before the loop starts. 1154 A convenient way to do this is like so:</p> 1155 1156 <div class="doc_code"> 1157 <pre> 1158 BasicBlock *BB = ... 1159 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I) 1160 ... use I ... 1161 </pre> 1162 </div> 1163 1164 <p>The observant may quickly point out that these two loops may have different 1165 semantics: if the container (a basic block in this case) is being mutated, then 1166 "<tt>BB->end()</tt>" may change its value every time through the loop and the 1167 second loop may not in fact be correct. If you actually do depend on this 1168 behavior, please write the loop in the first form and add a comment indicating 1169 that you did it intentionally.</p> 1170 1171 <p>Why do we prefer the second form (when correct)? Writing the loop in the 1172 first form has two problems. First it may be less efficient than evaluating it 1173 at the start of the loop. In this case, the cost is probably minor — a 1174 few extra loads every time through the loop. However, if the base expression is 1175 more complex, then the cost can rise quickly. I've seen loops where the end 1176 expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map 1177 lookups really aren't cheap. By writing it in the second form consistently, you 1178 eliminate the issue entirely and don't even have to think about it.</p> 1179 1180 <p>The second (even bigger) issue is that writing the loop in the first form 1181 hints to the reader that the loop is mutating the container (a fact that a 1182 comment would handily confirm!). If you write the loop in the second form, it 1183 is immediately obvious without even looking at the body of the loop that the 1184 container isn't being modified, which makes it easier to read the code and 1185 understand what it does.</p> 1186 1187 <p>While the second form of the loop is a few extra keystrokes, we do strongly 1188 prefer it.</p> 1189 1190 </div> 1191 1192 <!-- _______________________________________________________________________ --> 1193 <h4> 1194 <a name="ll_iostream"><tt>#include <iostream></tt> is Forbidden</a> 1195 </h4> 1196 1197 <div> 1198 1199 <p>The use of <tt>#include <iostream></tt> in library files is 1200 hereby <b><em>forbidden</em></b>, because many common implementations 1201 transparently inject a <a href="#ci_static_ctors">static constructor</a> into 1202 every translation unit that includes it.</p> 1203 1204 <p>Note that using the other stream headers (<tt><sstream></tt> for 1205 example) is not problematic in this regard — 1206 just <tt><iostream></tt>. However, <tt>raw_ostream</tt> provides various 1207 APIs that are better performing for almost every use than <tt>std::ostream</tt> 1208 style APIs. <b>Therefore new code should always 1209 use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> for writing, or 1210 the <tt>llvm::MemoryBuffer</tt> API for reading files.</b></p> 1211 1212 </div> 1213 1214 1215 <!-- _______________________________________________________________________ --> 1216 <h4> 1217 <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a> 1218 </h4> 1219 1220 <div> 1221 1222 <p>LLVM includes a lightweight, simple, and efficient stream implementation 1223 in <tt>llvm/Support/raw_ostream.h</tt>, which provides all of the common 1224 features of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> 1225 instead of <tt>ostream</tt>.</p> 1226 1227 <p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can 1228 be forward declared as <tt>class raw_ostream</tt>. Public headers should 1229 generally not include the <tt>raw_ostream</tt> header, but use forward 1230 declarations and constant references to <tt>raw_ostream</tt> instances.</p> 1231 1232 </div> 1233 1234 1235 <!-- _______________________________________________________________________ --> 1236 <h4> 1237 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a> 1238 </h4> 1239 1240 <div> 1241 1242 <p>The <tt>std::endl</tt> modifier, when used with <tt>iostreams</tt> outputs a 1243 newline to the output stream specified. In addition to doing this, however, it 1244 also flushes the output stream. In other words, these are equivalent:</p> 1245 1246 <div class="doc_code"> 1247 <pre> 1248 std::cout << std::endl; 1249 std::cout << '\n' << std::flush; 1250 </pre> 1251 </div> 1252 1253 <p>Most of the time, you probably have no reason to flush the output stream, so 1254 it's better to use a literal <tt>'\n'</tt>.</p> 1255 1256 </div> 1257 1258 </div> 1259 1260 <!-- ======================================================================= --> 1261 <h3> 1262 <a name="nano">Microscopic Details</a> 1263 </h3> 1264 <!-- ======================================================================= --> 1265 1266 <div> 1267 1268 <p>This section describes preferred low-level formatting guidelines along with 1269 reasoning on why we prefer them.</p> 1270 1271 <!-- _______________________________________________________________________ --> 1272 <h4> 1273 <a name="micro_spaceparen">Spaces Before Parentheses</a> 1274 </h4> 1275 1276 <div> 1277 1278 <p>We prefer to put a space before an open parenthesis only in control flow 1279 statements, but not in normal function call expressions and function-like 1280 macros. For example, this is good:</p> 1281 1282 <div class="doc_code"> 1283 <pre> 1284 <b>if (</b>x) ... 1285 <b>for (</b>i = 0; i != 100; ++i) ... 1286 <b>while (</b>llvm_rocks) ... 1287 1288 <b>somefunc(</b>42); 1289 <b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me"); 1290 1291 a = <b>foo(</b>42, 92) + <b>bar(</b>x); 1292 </pre> 1293 </div> 1294 1295 <p>and this is bad:</p> 1296 1297 <div class="doc_code"> 1298 <pre> 1299 <b>if(</b>x) ... 1300 <b>for(</b>i = 0; i != 100; ++i) ... 1301 <b>while(</b>llvm_rocks) ... 1302 1303 <b>somefunc (</b>42); 1304 <b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me"); 1305 1306 a = <b>foo (</b>42, 92) + <b>bar (</b>x); 1307 </pre> 1308 </div> 1309 1310 <p>The reason for doing this is not completely arbitrary. This style makes 1311 control flow operators stand out more, and makes expressions flow better. The 1312 function call operator binds very tightly as a postfix operator. Putting a 1313 space after a function name (as in the last example) makes it appear that the 1314 code might bind the arguments of the left-hand-side of a binary operator with 1315 the argument list of a function and the name of the right side. More 1316 specifically, it is easy to misread the "a" example as:</p> 1317 1318 <div class="doc_code"> 1319 <pre> 1320 a = foo <b>(</b>(42, 92) + bar<b>)</b> (x); 1321 </pre> 1322 </div> 1323 1324 <p>when skimming through the code. By avoiding a space in a function, we avoid 1325 this misinterpretation.</p> 1326 1327 </div> 1328 1329 <!-- _______________________________________________________________________ --> 1330 <h4> 1331 <a name="micro_preincrement">Prefer Preincrement</a> 1332 </h4> 1333 1334 <div> 1335 1336 <p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than 1337 postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use 1338 preincrementation whenever possible.</p> 1339 1340 <p>The semantics of postincrement include making a copy of the value being 1341 incremented, returning it, and then preincrementing the "work value". For 1342 primitive types, this isn't a big deal... but for iterators, it can be a huge 1343 issue (for example, some iterators contains stack and set objects in them... 1344 copying an iterator could invoke the copy ctor's of these as well). In general, 1345 get in the habit of always using preincrement, and you won't have a problem.</p> 1346 1347 </div> 1348 1349 <!-- _______________________________________________________________________ --> 1350 <h4> 1351 <a name="micro_namespaceindent">Namespace Indentation</a> 1352 </h4> 1353 1354 <div> 1355 1356 <p> 1357 In general, we strive to reduce indentation wherever possible. This is useful 1358 because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without 1359 wrapping horribly, but also because it makes it easier to understand the code. 1360 Namespaces are a funny thing: they are often large, and we often desire to put 1361 lots of stuff into them (so they can be large). Other times they are tiny, 1362 because they just hold an enum or something similar. In order to balance this, 1363 we use different approaches for small versus large namespaces. 1364 </p> 1365 1366 <p> 1367 If a namespace definition is small and <em>easily</em> fits on a screen (say, 1368 less than 35 lines of code), then you should indent its body. Here's an 1369 example: 1370 </p> 1371 1372 <div class="doc_code"> 1373 <pre> 1374 namespace llvm { 1375 namespace X86 { 1376 /// RelocationType - An enum for the x86 relocation codes. Note that 1377 /// the terminology here doesn't follow x86 convention - word means 1378 /// 32-bit and dword means 64-bit. 1379 enum RelocationType { 1380 /// reloc_pcrel_word - PC relative relocation, add the relocated value to 1381 /// the value already in memory, after we adjust it for where the PC is. 1382 reloc_pcrel_word = 0, 1383 1384 /// reloc_picrel_word - PIC base relative relocation, add the relocated 1385 /// value to the value already in memory, after we adjust it for where the 1386 /// PIC base is. 1387 reloc_picrel_word = 1, 1388 1389 /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just 1390 /// add the relocated value to the value already in memory. 1391 reloc_absolute_word = 2, 1392 reloc_absolute_dword = 3 1393 }; 1394 } 1395 } 1396 </pre> 1397 </div> 1398 1399 <p>Since the body is small, indenting adds value because it makes it very clear 1400 where the namespace starts and ends, and it is easy to take the whole thing in 1401 in one "gulp" when reading the code. If the blob of code in the namespace is 1402 larger (as it typically is in a header in the <tt>llvm</tt> or <tt>clang</tt> namespaces), do not 1403 indent the code, and add a comment indicating what namespace is being closed. 1404 For example:</p> 1405 1406 <div class="doc_code"> 1407 <pre> 1408 namespace llvm { 1409 namespace knowledge { 1410 1411 /// Grokable - This class represents things that Smith can have an intimate 1412 /// understanding of and contains the data associated with it. 1413 class Grokable { 1414 ... 1415 public: 1416 explicit Grokable() { ... } 1417 virtual ~Grokable() = 0; 1418 1419 ... 1420 1421 }; 1422 1423 } // end namespace knowledge 1424 } // end namespace llvm 1425 </pre> 1426 </div> 1427 1428 <p>Because the class is large, we don't expect that the reader can easily 1429 understand the entire concept in a glance, and the end of the file (where the 1430 namespaces end) may be a long ways away from the place they open. As such, 1431 indenting the contents of the namespace doesn't add any value, and detracts from 1432 the readability of the class. In these cases it is best to <em>not</em> indent 1433 the contents of the namespace.</p> 1434 1435 </div> 1436 1437 <!-- _______________________________________________________________________ --> 1438 <h4> 1439 <a name="micro_anonns">Anonymous Namespaces</a> 1440 </h4> 1441 1442 <div> 1443 1444 <p>After talking about namespaces in general, you may be wondering about 1445 anonymous namespaces in particular. 1446 Anonymous namespaces are a great language feature that tells the C++ compiler 1447 that the contents of the namespace are only visible within the current 1448 translation unit, allowing more aggressive optimization and eliminating the 1449 possibility of symbol name collisions. Anonymous namespaces are to C++ as 1450 "static" is to C functions and global variables. While "static" is available 1451 in C++, anonymous namespaces are more general: they can make entire classes 1452 private to a file.</p> 1453 1454 <p>The problem with anonymous namespaces is that they naturally want to 1455 encourage indentation of their body, and they reduce locality of reference: if 1456 you see a random function definition in a C++ file, it is easy to see if it is 1457 marked static, but seeing if it is in an anonymous namespace requires scanning 1458 a big chunk of the file.</p> 1459 1460 <p>Because of this, we have a simple guideline: make anonymous namespaces as 1461 small as possible, and only use them for class declarations. For example, this 1462 is good:</p> 1463 1464 <div class="doc_code"> 1465 <pre> 1466 <b>namespace {</b> 1467 class StringSort { 1468 ... 1469 public: 1470 StringSort(...) 1471 bool operator<(const char *RHS) const; 1472 }; 1473 <b>} // end anonymous namespace</b> 1474 1475 static void Helper() { 1476 ... 1477 } 1478 1479 bool StringSort::operator<(const char *RHS) const { 1480 ... 1481 } 1482 1483 </pre> 1484 </div> 1485 1486 <p>This is bad:</p> 1487 1488 1489 <div class="doc_code"> 1490 <pre> 1491 <b>namespace {</b> 1492 class StringSort { 1493 ... 1494 public: 1495 StringSort(...) 1496 bool operator<(const char *RHS) const; 1497 }; 1498 1499 void Helper() { 1500 ... 1501 } 1502 1503 bool StringSort::operator<(const char *RHS) const { 1504 ... 1505 } 1506 1507 <b>} // end anonymous namespace</b> 1508 1509 </pre> 1510 </div> 1511 1512 1513 <p>This is bad specifically because if you're looking at "Helper" in the middle 1514 of a large C++ file, that you have no immediate way to tell if it is local to 1515 the file. When it is marked static explicitly, this is immediately obvious. 1516 Also, there is no reason to enclose the definition of "operator<" in the 1517 namespace just because it was declared there. 1518 </p> 1519 1520 </div> 1521 1522 </div> 1523 1524 </div> 1525 1526 <!-- *********************************************************************** --> 1527 <h2> 1528 <a name="seealso">See Also</a> 1529 </h2> 1530 <!-- *********************************************************************** --> 1531 1532 <div> 1533 1534 <p>A lot of these comments and recommendations have been culled for other 1535 sources. Two particularly important books for our work are:</p> 1536 1537 <ol> 1538 1539 <li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective 1540 C++</a> by Scott Meyers. Also 1541 interesting and useful are "More Effective C++" and "Effective STL" by the same 1542 author.</li> 1543 1544 <li>Large-Scale C++ Software Design by John Lakos</li> 1545 1546 </ol> 1547 1548 <p>If you get some free time, and you haven't read them: do so, you might learn 1549 something.</p> 1550 1551 </div> 1552 1553 <!-- *********************************************************************** --> 1554 1555 <hr> 1556 <address> 1557 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 1558 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 1559 <a href="http://validator.w3.org/check/referer"><img 1560 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 1561 1562 <a href="mailto:sabre (a] nondot.org">Chris Lattner</a><br> 1563 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 1564 Last modified: $Date$ 1565 </address> 1566 1567 </body> 1568 </html> 1569
